A steering mechanism unit of an automobile has hitherto involved the use of, as a part of the steering mechanism unit, a telescopic shaft absorbing an axial displacement occurred when the automobile travels and including a male shaft and a female shaft that are spline-fitted to each other in order not to transfer the displacement and vibrations onto a steering wheel. What is required of the telescopic shaft is to reduce backlash noises at the spline portion, to reduce a feeling of backlash on the steering wheel and to reduce a slide resistance when sliding in axial directions.
For these requirements, the spline portion of the male shaft of the telescopic shaft is coated with a nylon layer, and further a grease is applied over a sliding portion, thereby absorbing or relieving metal noises, metal butting sounds, etc. and also reducing the slide resistance and the backlash in a rotating direction. In this case, a processing flow of forming the nylon layer is: cleaning the shaft→coating a primer→heating→coating nylon powder→rough cutting→finish-cutting→selective fitting to the female shaft. A final cutting work is conducted in a way that selects dies adjusted to an accuracy of the already-worked female shaft.
It is required that even the backlash be restrained to the minimum while restraining a slide load on the telescopic shaft to the minimum, and hence the final cutting work has no option but to select the dies matched with the female shaft which differ by every several microns in size of an over-pin diameter and to perform working, with the result that a rise in working cost is brought about. Moreover, the backlash in the rotating direction increases as an abrasion of the nylon layer progresses with transitions of the use.
Further, under such a condition as to be exposed to a high temperature in an engine room, the nylon layer changes in volume as followed by a remarkable rise in slide resistance and a remarkably-accelerated progression of the abrasion, and therefore the backlash in the rotating direction increases.
Accordingly, there is a demand for providing the telescopic shaft employed for the steering shaft for the automobile with a simple and low-cost structure capable of restraining over a long period of time an emission of heterogeneous sounds due to the backlash in the rotating direction and deterioration in the feeling of steering.